Thermosetting resins that are commonly used in fiber-reinforced composites cannot be reshaped after thermoforming. Errors in forming cannot be corrected, so these thermosetting resins are undesirable in many applications.
Although thermoplastic resins are well known, the use of fiber-reinforced thermoplastic resins is a relatively new art. Fiber toughens and stiffens the thermoplastic resin to produce high-performance composite products. A sheet of fiber-reinforced resin can be heated and then stamped into a desired shape with appropriate dies. The shape can be altered thereafter, if desired.
Thermoplastic resins commonly have a tendency to be weakened by organic solvents. Accordingly, circuit boards formed from conventional, fiber-reinforced thermoplastic resin composites usually cannot be cleaned with solvents that are commonly used in the aerospace industry. In structural aircraft applications, care must also be taken to eliminate contact between the composites and hydraulic or cleaning fluids. At moderate or high temperatures, many fiber-reinforced thermoplastic composites lose their abilities to carry load due to softening of the resin. Thus, improved thermal stability and solvent-resistance are desirable to fulfill the existing needs for advanced composites. The oligomers of the present invention provide such polyimide composites when they are cured.
Recently, chemists have sought to synthesize oligomers for high performance advanced composites suitable for aerospace applications. These composites should exhibit solvent resistance, toughness, impact resistance, ease of processing, and strength, and should be thermoplastic. Oligomers and composites that have thermooxidative stability and, accordingly, can be used at elevated temperatures are particularly desirable. They are essential for the fabrication of the next generation supersonic military or commercial transport.
While epoxy-based composites are suitable for many applications, their brittle nature and susceptibility to degradation make them inadequate for many aerospace applications, especially those applications which require thermally stable, tough composites. Accordingly, research has recently focused on polyimide composites to achieve an acceptable balance between thermal stability, solvent resistance, and toughness. Still the maximum temperatures for use of the polyimide composites, such as PMR-15, are about 600.degree.-625.degree. F., since they have glass transition temperatures of about 690.degree. F. These formulations present manufacturing obsticles for reproducibility and reliability because of prepreg aging and also pose health and safety concerns with carcinogens mixed in the monomer reactants prior to cure.
There has been a progression of polyimide sulfone compounds synthesized to provide unique properties or combinations of properties. For example, Kwiatkowski and Brode synthesized maleic capped linear polyarylimides as disclosed in U.S. Pat. No. 3,839,287. Holub and Evans synthesized maleic or nadic capped imido-substituted polyester compositions as disclosed in U.S. Pat. No. 3,729,446. Monacelli proposed tetra-maleimides made through an amic acid mechanism with subsequent ring closure, as shown in U.S. Pat. Nos. 4,438,280 or 4,418,181. We synthesized thermally stable polysulfone oligomers (U.S. Pat. No. 4,476,184 or U.S. Pat. No. 4,536,559) polyimidesulfones that are fully-imidized yet soluble in conventional processing solvents (U.S. Pat. Nos. 5,001,905 or 5,175,234) polybenzoxazolesulfones, polybutadienesulfones, and "star" or "star-burst" multidimensional oligomers. We have shown surprisingly high glass transition temperatures yet reasonable processing and desirable physical properties in many of these oligomers and their composites.
Polybenzoxazoles (or their corresponding heterocycles), such as those disclosed in our U.S. Pat. Nos. 4,965,336 and 4,868,270 may be used at temperatures up to about 750.degree.-775.degree. F., since these composites have glass transition temperatures of about 840.degree. F. Some aerospace applications need composites which have even higher use temperatures while maintaining toughness, solvent resistance, ease of processing, formability, strength, and impact resistance.
Multidimensional oligomers, such as disclosed in U.S. Pat. No. 5,210,213 or our copending U.S. patent application Ser. Nos. 07/167,656 and 07/176,518, have superior processibility than many other advanced oligomers since they can be processed at lower temperatures. Upon curing, however, the phenylimide end caps crosslink so that the thermal resistance and stiffness of the resulting composite is markedly increased. This increase is in thermomechanical and thermo-oxidative stability obtained with only a minor loss of matrix stress transfer (impact resistance), toughness, elasticity, and other mechanical properties, and can achieve glass transition temperatures above 850.degree. F.
Commercial polyesters, when combined with well-known reactive diluents, such as styrene, exhibits marginal thermal and oxidative resistance, and are useful only for aircraft or aerospace interiors. Polyarylesters are often unsatisfactory, also, since the resins often are semicrystalline which may make them insoluble in usable laminating solvents, intractable in fusion under typical processing conditions, and difficult and expensive to manufacture because of shrinking and/or warping. Those polyarylesters that are soluble in conventional laminating solvents remain so in composite form, thereby limiting their usefulness in structural composites. The high concentration of ester groups contributes to resin Strength and tenacity, but also to make the resin susceptible to the damaging effects of water absorption. High moisture absorption by commercial polyesters can lead to lowering of the glass transition temperature leading to distortion of the composite when it is loaded at elevated temperature.
High performance, aerospace, polyester advanced composites, however, can be prepared using crosslinkable, end-capped polyester imide ether sulfone oligomers that have an acceptable combination of solvent resistance, toughness, impact resistance, strength, ease of processing, formability, and thermal resistance. By including Schiff base (--CH.dbd.N--) linkages in the oligomer chain, the linear, advanced composites formed with polyester oligomers of our copending application U.S. Ser. No. 07/137,493 can have semiconductive or conductive properties when appropriately doped or reacted with appropriate metal salts.
Conductive and semiconductive plastics have been extensively studied (see, e.g., U.S. Pat. Nos. 4,375,427; 4,338,222; 3,966,987; 4,344,869; and 4,344,870), but these polymers do not possess the blend of properties which are essential for aerospace applications. That is, the conductive polymers do not possess the blend of (1) toughness, (2) stiffness, (3) ease of processing, (4) impact resistance (and other matrix stress transfer capabilities), (5) retention of properties over a broad range of temperatures, and (6) thermooxidative resistance that is desirable on aerospace advanced composites. The prior art composites are often too brittle.
Thermally stable multidimensional oligomers having semiconductive or conductive properties when doped with suitable dopants are also known and are described in our copending applications (including U.S. patent application Ser. Nos. 06/773,381 and 07/212,404). The linear arms of the oligomers contain conductive linkages, such as Schiff base (--N.dbd.CH--) linkages, between aromatic groups. Sulfone and ether linkages are interspersed in the arms. Each arm is terminated with a mono- or difunctional end cap to allow controlled crosslinking upon heat-induced or chemical-induced curing.
For imides and many other resin backbones, we have shown surprisingly high glass transition temperatures, reasonable processing parameters and constraints for making and using the prepregs, and desirable physical properties for the composites by using soluble oligomers having difunctional caps, especially those with nadic caps. Linear oligomers of this type include two crosslinking functionalities at each end of the resin chain to promote crosslinking upon curing. Linear oligomers are "monofunctional" when they have one crosslinking functionality at each end. The preferred linear oligomers from our earlier research are "difunctional," because they had two functional groups at each end. Upon curing, the crosslinking functionalities provide sites for chain extension. Similarly, multidimensional oligomers are difunctional when each arm terminates with two caps. Because the crosslinks were generally the weakest portions of the resulting composite, we improved thermo-oxidative stability of the composites by including two crosslinks at each junction. We built in redundancy, then, at each weak point. We maintained solubility of the reactants and resins using, primarily, phenoxyphenyl sulfone chemistries. Our work during the past fifteen years across a broad range of resin types or chemical families is described in the following, forty-nine U.S. Patents (all of which we incorporate by reference):
__________________________________________________________________________ INVENTOR U.S. Pat. No. TITLE ISSUE DATE __________________________________________________________________________ Lubowitz et al. 4,414,269 Solvent Resistant Poly- November 8, 1983 sulfone and Polyether- sulfone Composites Lubowitz et al. 4,476,184 Thermally Stable Poly- October 9, 1984 sulfone Compositions for Composite Structures Lubowitz et al. 4,536,559 Thermally Stable Polyimide August 20, 1985 Polysulfone Compositions for Composite Structures Lubowitz et al. 4,547,553 Polybutadiene Modified October 15, 1985 Polyester Compositions Lubowitz et al. 4,584,364 Phenolic-Capped Imide April 22, 1986 Sulfone Resins Lubowitz et al. 4,661,604 Monofunctional Cross- April 28, 1987 linking Imidophenols Lubowitz et al. 4,684,714 Method for Making August 4, 1987 Polyimide Oligomers Lubowitz et al. 4,739,030 Difunctional End-Cap April 19, 1988 Monomers Lubowitz et al. 4,847,333 Blended Polyamide July 11, 1989 Oligomers Lubowitz et al. 4,851,495 Polyetherimide Oligomers July 25, 1989 Lubowitz et al. 4,851,501 Polyethersulfone Prepregs, July 25, 1989 Composites, and Blends Lubowitz et al. 4,868,270 Heterocycle Sulfone September 19, 1989 Oligomers and Blends Lubowitz et al. 4,871,475 Method for Making October 3, 1989 Polysulfone and Polyethersulfone Oligomers Lubowitz et al. 4,876,328 Polyamide Oligomers October 24, 1989 Lubowitz et al. 4,935,523 Crosslinking June 19, 1990 Imidophenylamines Lubowitz et al. 4,958,031 Crosslinking September 18, 1990 Nitromonomers Lubowitz et al. 4,965,336 High Performance October 23, 1990 Heterocycle Oligomers and Blends Lubowitz et al. 4,980,481 Pyrimidine-Based End-Cap December 25, 1990 Monomers and Oligomers Lubowitz et al. 4,981,922 Blended Etherimide January 1, 1991 Oligomers Lubowitz et al. 4,985,568 Method of Making January 15, 1991 Crosslinking Imidophenyl- amines Lubowitz et al. 4,990,624 Intermediate Anhydrides February 5, 1991 Useful for Synthesizing Etherimides Lubowitz et al. 5,011,905 Polyimide Oligomers and April 30, 1991 Blends Lubowitz et al. 5,066,541 Multidimensional November 19, 1991 Heterocycle Sulfone Oligomers Lubowitz et al. 5,071,941 Multidimensional Ether December 10, 1991 Sulfone Oligomers Lubowitz et al. 5,082,905 Blended Heterocycles January 21, 1992 Lubowitz et al. 5,087,701 Phthalimide Acid Halides February 11, 1992 Lubowitz et al. 5,104,967 Amideimide Oligomers and April 14, 1992 Blends Lubowitz et al. 5,109,105 Polyamides April 28, 1992 Lubowitz et al. 5,112,939 Oligomers Having May 12, 1992 Pyrimidinyl End Caps Lubowitz et al. 5,115,087 Coreactive Imido Oligomer May 19, 1992 Blends Lubowitz et al. 5,116,935 High Performance Modified May 26, 1992 Cyanate Oligomers and Blends Lubowitz et al. 5,120,819 High Performance June 9, 1992 Heterocycles Lubowitz et al. 5,126,410 Advanced Heterocycle June 30, 1992 Oligomers Lubowitz et al. 5,144,000 Method for Forming September 1, 1992 Crosslinking Oligomers Lubowitz et al. 5,151,487 Method of Preparing a September 29, 1992 Crosslinking Oligomer Lubowitz et al. 5,155,206 Amideimide Oligomers, October 13, 1992 Blends and Sizings for Carbon Fiber Compo-sites Lubowitz et al. 5,159,055 Coreactive Oligomer October 27, 1992 Blends Lubowitz et al. 5,175,233 Multidimensional Ester or December 29, 1992 Ether Oligomers with Pyrimidinyl End Caps Lubowitz et al. 5,175,234 Lightly-Crosslinked December 29, 1992 Polyimides Lubowitz et al. 5,175,304 Halo- or Nitro- December 29, 1992 Intermediates Useful for Synthesizing Etherimides Lubowitz et al. 5,198,526 Heterocycle Oligomers with March 30, 1993 Multidimensional Morphology Lubowitz et al. 5,210,213 Multidimensional May 11, 1993 Crosslinkable Oligomers Lubowitz et al. 5,216,117 Amideimide Blends June 1, 1993 Lubowitz et al. 5,227,461 Extended Difunctional End- July 13, 1993 Cap Monomers Lubowitz et al. Reissue 34,820 Amideimide Sizing For August 24, 1993 Carbon Fiber (originally issued) Lubowitz et al. 5,268,519 Lightly Crosslinked December 7, 1993 Etherimide Oligomers Lubowitz et al. 5,286,811 Blended Polyimide February 15, 1994 Oligomers and Method of Curing Polyimides Lubowitz et al. 5,344,894 Polyimide Oligomers and September 6, 1994 Blends Lubowitz 5,403,666 Composites Containing April 4, 1995 Amideimide Sized Fibers __________________________________________________________________________
We recently described advanced oligoners that include as many as four caps at each end of the chains in U.S. patent application Ser. Nos. 08/327,942 and 08/327,180. Again, we add further redundancy at the weakest link in the cured composite. We also obtain micelles within the composite for increased compressive strength. Nevertheless, we have the increased processing questions from four reactive caps at each crosslinking site, making it more important that our processing window provide alequate flow in the curing cycle.
High speed aircraft, particularly the High Speed Civil Transport (HSCT), will likely be constructed from advanced composites. The flight regime for these commercial aircraft, however, impose difficult requirements on the materials. At the proposed supersonic speeds (of about Mach 2.2), the skin of the aircraft will be exposed to extreme heat. For success, the composites must withstand long exposure to such temperatures the composites will face thermal cycling for ascent and decent. If possible, the composites should have useful lives of about 120,000 hours. These requirements challenge all known materials. The polyimide oligomers of the present invention, nevertheless, are candidates to meet this challenge in the next century.